diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0cbc5bdc..2610db92 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,6 +24,13 @@ FlatLaf Change Log
   invocation).
 - macOS: Fixed crash when running in WebSwing. (issue #826; regression in 3.4)
 
+#### Incompatibilities
+
+- File names of custom properties files for nested Laf classes now must include
+  name of enclosing class name. E.g. nested Laf class `IntelliJTheme.ThemeLaf`
+  used `ThemeLaf.properties` in previous versions, but now needs to be named
+  `IntelliJTheme$ThemeLaf.properties`.
+
 
 ## 3.4
 
diff --git a/flatlaf-core/src/main/java/com/formdev/flatlaf/FlatDefaultsAddon.java b/flatlaf-core/src/main/java/com/formdev/flatlaf/FlatDefaultsAddon.java
index ac73cff9..f118d701 100644
--- a/flatlaf-core/src/main/java/com/formdev/flatlaf/FlatDefaultsAddon.java
+++ b/flatlaf-core/src/main/java/com/formdev/flatlaf/FlatDefaultsAddon.java
@@ -48,7 +48,7 @@ public abstract class FlatDefaultsAddon
 	public InputStream getDefaults( Class<?> lafClass ) {
 		Class<?> addonClass = this.getClass();
 		String propertiesName = '/' + addonClass.getPackage().getName().replace( '.', '/' )
-			+ '/' + lafClass.getSimpleName() + ".properties";
+			+ '/' + UIDefaultsLoader.simpleClassName( lafClass ) + ".properties";
 		return addonClass.getResourceAsStream( propertiesName );
 	}
 
diff --git a/flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java b/flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java
index 79345841..af21c3d0 100644
--- a/flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java
+++ b/flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java
@@ -161,7 +161,7 @@ class UIDefaultsLoader
 						classLoader = FlatLaf.class.getClassLoader();
 
 					for( Class<?> lafClass : lafClasses ) {
-						String propertiesName = packageName + '/' + lafClass.getSimpleName() + ".properties";
+						String propertiesName = packageName + '/' + simpleClassName( lafClass ) + ".properties";
 						try( InputStream in = classLoader.getResourceAsStream( propertiesName ) ) {
 							if( in != null )
 								properties.load( in );
@@ -171,7 +171,7 @@ class UIDefaultsLoader
 					// load from package URL
 					URL packageUrl = (URL) source;
 					for( Class<?> lafClass : lafClasses ) {
-						URL propertiesUrl = new URL( packageUrl + lafClass.getSimpleName() + ".properties" );
+						URL propertiesUrl = new URL( packageUrl + simpleClassName( lafClass ) + ".properties" );
 
 						try( InputStream in = propertiesUrl.openStream() ) {
 							properties.load( in );
@@ -183,7 +183,7 @@ class UIDefaultsLoader
 					// load from folder
 					File folder = (File) source;
 					for( Class<?> lafClass : lafClasses ) {
-						File propertiesFile = new File( folder, lafClass.getSimpleName() + ".properties" );
+						File propertiesFile = new File( folder, simpleClassName( lafClass ) + ".properties" );
 						if( !propertiesFile.isFile() )
 							continue;
 
@@ -294,6 +294,14 @@ class UIDefaultsLoader
 		}
 	}
 
+	/**
+	 * Similar to Class.getSimpleName(), but includes enclosing class for nested classes.
+	 */
+	static String simpleClassName( Class<?> cls ) {
+		String className = cls.getName();
+		return className.substring( className.lastIndexOf( '.' ) + 1 );
+	}
+
 	static void logParseError( String key, String value, RuntimeException ex, boolean severe ) {
 		String message = "FlatLaf: Failed to parse: '" + key + '=' + value + '\'';
 		if( severe )